The trend for a long while has been to make text on a screen larger and easier to read for all users. In the past, we used tons of small typefaces that might have looked cool but were hard to read for anyone visually challenged. To that end, the base font-size for Bootstrap has been changed from 14px to 16px. This is also the standard size for most browsers and makes the readability of text better. Again, from a migration standpoint, you'll need to review your components to ensure they still look correct with the increased font size. You may need to make some changes if you have components that were based on the 14px default font-size in Bootstrap 3.
With the increased use of mobile devices, Bootstrap 4 includes a new smaller grid tier for small screen devices. The new grid tier is called extra small and is configured for devices under 480px in width. For the migration story this shouldn't have a big effect. What it does do is allow you a new breakpoint if...